Laboratory evaluation of patients with suspected systemic mastocytosis (SM) includes: (1) morphologic review of bone marrow biopsies and aspirates, (2) molecular testing for the presence of the KIT mutation and (3) immunophenotypic analysis of mast cells and other hematopoietic elements by flow cytometry. Recently, we have developed RT-PCR/RFLP assay for detection of the most prevalent KIT mutation in mastocytosis patients (D816V).The KIT D816V mutation results in constitutive activation of the receptor tyrosine kinase and is believed to be involved in disease pathogenesis. We have also developed a flow cytometric assay for immunophenotypic analysis of mast cells, since it has been recently described that neoplastic mast cells display aberrant immunophenotype, most notably, aberrantly coexpress CD117 with CD25 and/or CD2. Aberrant expression of CD25 and CD2 is not detected in normal marrow mast cells. [unreadable] The identification of the KIT D816V mutation in patients with systemic mastocytosis has lately gained a major prognostic significance, largely because of the availability of tyrosine kinase receptor inhibitors such as imatinib. Imatinib was shown to be ineffective in patients carrying KIT D816V mutation, but effective in cases with some other mutations. Therefore, it is of paramount importance to correctly identify SM patients with KIT D816V mutation. Although most patients with SM do not have peripheral blood eosinophilia, both bone marrow and peripheral blood eosinophilia have been reported in D816V mutation-positive patients. In these cases, the broad and overlapping clinical manifestations between the patients with D816V KIT-associated systemic mastocytosis with eosinophilia and patients with chronic eosinophilic leukemia (CEL) associated with FIP1L1/PDGFRA fusion gene present diagnostic challenge. The increase in activated eosinophils and mast cells is seen in both disorders, and has led to confusion in the nomenclature. It is of paramount importance, however, to distinguish between these two groups of patients because of differences in clinical sequelae, prognoses and selection of treatment. This has profound consequences for patients with these disorders, since the FIP1L1/PDGFRA and D816V mutations respond dramatically differently to therapy. Most notably, the FIP1L1/PDGFRA fusion kinase, but not D816V KIT mutation, is inhibited by imatinib. We thus sought to identify clinical and laboratory features that could be used to distinguish these two diagnoses. We compared 12 patients with D816V-positive systemic mastocytosis with eosinophilia with 17 patients with FIP1L1/PDGFRA-positive CEL. Distinguishing features were used to create a risk factor scoring system. This system correctly classified 16/17 FIP1L1/PDGFRA-positive CEL patients, and all 12 systemic mastocytosis with eosinophilia patients. These results reinforce the hypothesis that the FIP1L1/PDGFRA gene fusion and D816V-KIT mutation cause distinct clinical syndromes. This novel diagnostic approach should prove helpful in clinical practice in the evaluation of patients with increased mast cells burden and peripheral eosinophilia.[unreadable] [unreadable] Detection of other molecular/cytogenetic abnormalities in SM patients is much less frequent. We identified an unusual case of a patient presenting with peripheral basophilia and systemic mastocytosis in whom cytogenetic analysis revealed a t(4;5)(q21.1;q31.3).[unreadable] Translocations involving region 5q31-32 (PDGFRB) have been reported in a variety of myeloproliferative diseases and are often associated with significant peripheral eosinophilia. We used molecular analysis to determine the role of PDGFRB in this case. Fluorescence in situ hybridization (FISH) documented a breakpoint in PDGFRB. In agreement with this, the patient responded very well to imatinib treatment with resolution of clinical symptoms, basophilia, and mast cell disease. Molecular analysis revealed that PDGFRB, encoding an imatinib-sensitive tyrosine kinase, was fused to PRKG2. The fusion gene incorporates the first two exons of PRKG2 fused to the truncated exon 12 of PDGFRB, resulting in the disruption of its juxtamembrane domain. Functional studies confirmed that the activity and transforming properties of PRKG2-PDGFRbeta were dependent on the disruption of the auto-inhibitory juxtamembrane domain. Our results identify a second case of the PRKG2-PDGFRB fusion and confirm the unusual PDGFRB breakpoint associated with this fusion. This work also illustrates the use of imatinib for the treatment of specific cases of systemic mastocytosis.